1. Field of the Invention
The invention generally relates to chemical analysis instruments and, more particularly, to devices for monitoring the amount of material dissolved in a liquid solution.
2. Description of the Prior Art
Proper determination of the amount of material dissolved in a liquid is a problem frequently encountered in chemical processing facilities. For example, in the printed circuit board industry there is a need to know the amount of photoresist dissolved in a developer or stripper bath for replacement or replenishment purposes, in waste water management facilities there is a need to know the amount of dissolved solids in water effluent to satisfy health code regulations, in an electroplating operation there is a need to know the clarity of solution because it is an indication of the amount of plating chemical in solution. Other chemical processing facilities have similar solution monitoring requirements. Visual estimation is inaccurate at best and totally wrong at worst. Laboratory determination of the percentage of dissolved solids is inconvenient, time consuming and expensive. However, the penalty for having too much or too little material in a solution can be serious.
Currently, an emphasis has been placed on fully automating chemical processing facilities. Decreased human involvement can reduce costs and improve plant safety. For example, in the printed circuit manufacturing business, the operation of printed circuit printers can be improved by monitoring systems which provide automatic checks on the developer and stripper baths. Printed circuit printing involves the use of photoresist material for various operations. Photoresist which is exposed to ultraviolet (UV) energy is polymerized and is removed from the printed circuit substrate or laminate with a stripper solution (typically, sodium hydroxide). Photoresist which is protected from UV induced polymerization is removed from the printed circuit substrate or laminate with a developer solution (typically, 1-2% sodium carbonate). In both developing and stripping, the developer or stripper solution is sprayed over the substrate or laminate and the photoresist enters semi-solution in the developer or stripper bath, respectively. Photoresist collecting in the developer or stripper baths is called "solution loading".
Proper use of developer and stripper solutions is a compromise between the volume of developer and stripper solutions used and the cleanliness of the developer and stripper solutions. It would be desirable to use perfectly clean developer and stripper solutions for each substrate being patterned; however, large amounts of waste developer and stripper would be produced if clean developer and stripper was used for every substrate. Operating costs are increased by the added cost of proper treatment of waste developer and stripper. Allowing the developer or stripper solution to dissolve excessive amounts of photoresist ultimately results in very slow or improper developing or stripping. Improper development may lead to between-trace shorts which render the printed circuit produced useless.
In prior art printed circuit printers, the following methods for changing or replenishing developer and stripper solutions have been used: (1) the solution is changed routinely on a fixed time schedule, such as once per day, (2) the board area processed is estimated and replenishment or replacement is implemented when solution loading approaches a predetermined level, (3) samples are evaluated in a chemistry lab on a regular basis and replenishment or replacement occurs when loading approaches a predetermined level, and (4) the solution pH is measured and replenishment or replacement occurs when the pH drops below a predetermined level. None of these approaches has provided totally satisfactory results. In methods (1) and (2), replenishing or replacing the solution occurs without testing the solution; therefore, it is not possible to determine whether a proper compromise between the volume of developer or stripper used and the cleanliness of the developer or stripper has been reached. Method (3) requires personnel to periodically dip a sample cup into the bath and then analyze the level of photoresist solids in the sample. In addition to the salaries required for trained personnel and costs associated with testing instruments such as spectrophotometers, periodic sampling has the additional disadvantages of being too time consuming to accommodate the quick and easy results demanded in a production environment and of putting too much reliance on the personnel to take the samples at properly timed intervals. Only method (4), measurements based on pH, is compatible with automated operation; however, because of various technical problems, results using pH as a guide have not been very satisfactory.
Printed circuit printers use medium intensity ultraviolet (UV) lamps for imaging a substrate clad with photoresist. Proper exposure of the photoresist is very important to overall process quality. UV integrators equipped with silicon photodetector UV sensors control the amount of UV energy which is directed upon the photoresist. In present day printed circuit printers, the UV integrator and associated sensor are used continuously during normal operation. Sensors have been found to experience long term degradation due to prolonged exposure to UV energy (a phenomena called "solarization"). The integrator circuitry itself is also subject to drift and occasional failure.